API should return 400 error if attempting to delete file that doesn't exist

[madprime]

Our API endpoint for file deletion (/api/direct-sharing/project/files/delete/) should return a 400 error if no files exist that can be deleted.

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Description

Currently we have a server error when a project attempts to delete a file by file_id, and that file no longer exists. See: https://github.com/OpenHumans/open-humans/blob/master/private_sharing/api_views.py#L332

This should be handled in the code and return a 400 error in the API, something like "No file found matching this ID."

Also, requests made by file_basename should produce an error if no matching file exists.